The present invention relates generally to a method and apparatus for coating a fabric with elastomeric stock, wherein improved impregnation of the interstices of the fabric is achieved, and more particularly to a method and apparatus in which there is utilized an assembly constituted primarily of a driven roller and a stationary dieblade confronting the roller and defining therewith a convergent nozzle-like chamber. The chamber surfaces operate on elastomeric stock to cause the latter to increasingly impregnate a fabric fed concomitantly therewith between the driven roller and stationary die-blade as both the elastomeric stock and fabric are commonly wedged and subjected to increasing pressure along their common direction of feed.
Conventional practices for manufacturing or otherwise forming elastomeric products, in the nature of conveyor belts and the like, generally employ a procedure in which the fabric substance of the product is coated or impregnated by an elastomeric stock material such as natural rubber, synthetic resins of thermoplastic nature, combinations of natural rubber and synthetic resins, or any other suitable plasticized composition generally utilized therefor, the coating or impregnation being effected by a pair of opposed rolls of a conventional calender apparatus.
In this respect, the fabric is fed between and through the opposed rolls of the calender, one or both of the opposed or primary rolls being associated with respective secondary rolls which provide means for banking excess elastomeric stock upon the primary rolls so that the primary rolls may, upon contacting the opposite surfaces of the fabric, transfer the elastomeric coating thereon onto one or both of the opposite fabric surfaces.
The degree of impregnation of the fabric by the elastomeric stock is primarily dependent upon, or is a function of, the degree of pressure that the opposed calender rolls can impart to the elastomeric stock and fabric as the latter pass through the nip therebetween. As is well understood, conventional calender rolls permit means for altering or increasing the extent of pressure imparted to the elastomeric stock and fabric such as by banking, in a manner aforementioned, or by increasing the peripheral velocity of the rolls, or by decreasing the clearance or nip between the opposed rolls so that the respective periphery of each of the rolls exerts a greater pressure against the opposite surfaces of the elastomeric stock and fabric as the latter pass therebetween.
The disadvantage associated with merely relying upon the capacity for increasing the roll velocity or narrowing the nip clearance is that the product may be insufficiently coated or unduly thin as formed. Alternatively, the fabric may be torn due to excessive stress as the elastomeric stock is transferred thereto. Thus, there is a limitation to the extent to which the roll velocity can be increased or the clearance between the calender rolls may be reduced. This is a severe restriction on the degree of pressure that can be utilized for transferring the elastomeric stock from the calender rolls to the opposite surfaces of the fabric for purposes of coating and impregnating the latter.
Moreover, the nip between conventional calender rolls provides only a minimum effective or working surface area with which the opposite elastomeric stock and fabric surfaces coact, the effective or working surface area being respective segments of the opposing rolls of minimal arcuate extent which converge toward and thereafter diverge away from one another. Thus, only an extremely short segment of each of the opposed rolls contacts the opposite surfaces of the elastomeric stock and fabric during transferring of the elastomeric stock to the fabric surfaces. This results in a minimal residence period during which the elastomeric stock and fabric are under pressure by the opposing rolls. It also results in a lesser degree of impregnation of the fabric by the elastomeric stock than may be most desirable under the circumstances, and in a lesser capacity to resist or retard swelling of the stock after the latter and fabric issue from between the rolls.
Therefore, although calender rolls permit the increasing of nip pressures such as by banking or otherwise building up of excess elastomeric stock upon each of the calender rolls, or by increasing the peripheral velocity of the rolls, or by decreasing the nip clearance between the rolls, the extent of increasing the pressure in this manner is still limited. Moreover the effective extent of each of the arcuate working surface segments of the opposing rolls is self-limiting and thus, the degree of impregnation of the fabric by the elastomeric material, and swell-retardation of the elastomeric material after coating, remains limited.